The present invention is related to a communication transformer employed in a LAN, or the like, in which a non-shielded twisted pair cable is used as a transmission path so as to perform a communicating operation. More specifically, the present invention is directed to a communication transformer which is employed in a power line communication, while the power line is used as a communication medium.
Recently, non-shielded type twisted pair cables are mainly utilized as transfer media for LANs (Local Area Networks). In connection with high transfer speeds, there is a problem that noise is radiated from the above-described transfer media. More particularly, in electric power line communications in which electric power lines are employed as transfer media, since such media (namely, electric power line) which has not been originally designed as communication paths is used as these communication paths, noise suppressing solutions are strongly requested.
In general, common-mode noise mainly constitutes noise radiation from balanced transfer paths such as twisted pair cables and parallel electric cables, while the common-mode noise is produced between the transfer paths and the ground. As a result, signals have been transmitted to the transfer paths via communication transformers having superior common-mode noise rejection performance. To this end, such a method has been conceived. That is, a winding which constitutes a shielded line is added between windings of a communication transformer, so that common-mode rejection performance of the communication transformer is improved (refer to, for example, Japanese Laid-open Patent Application No. Hei-7-192962).
FIG. 14 is a structural diagram of a communication transformer in the conventional technique. That is, a pulse transformer (broad-band communication transformer) which is used in a communication module for 10BASE-T has been described. FIG. 15 is an equivalent circuit diagram of the communication transformer in the prior art. A primary winding 16, a secondary winding 17, and an additional winding 18 are wound on a core 15, while any one of additional winding terminals 19 and 20 of the additional winding 18 is grounded. Since such an arrangement is employed, stray capacitances produced between the primary winding 16 and the secondary winding 17 are reduced so as to decrease the common-mode noise which is propagated between the primary winding 16 and the secondary winding 17.
However, in the conventional method, there is such a problem that since the additional winding 18 is grounded, the electrical balance of the transformer is lowered. This reason is given as follows. That is, as shown in FIG. 15, an impedance of the primary winding terminal 21 with respect to the ground becomes such a series connection made of the stray capacitance 23 and a resistor 25 owned by the additional winding 18 itself, whereas an impedance as viewed from the terminal 22 to the ground constitutes only the stray capacitance 24. As a result, there is a difference in the impedances at the primary winding terminal 21 and the terminal 22 with respect to the ground.
FIG. 16 is a conversion transfer loss performance diagram of the conventional communication transformer, namely, is represented by a graph. A longitudinal-direction conversion transfer loss is expressed in which a ratio of a normal mode signal appearing on the secondary side to a common-mode signal applied to the primary-sided transfer-purpose winding is plotted every frequency. In an ideal transformer case, no normal mode signal appears on the secondary side of this ideal transformer. However, in an actual transformer, since the electrical balance becomes a finite value, such a normal mode signal which has been converted from the common-mode signal appears on the secondary side of the actual transformer. As a consequence, FIG. 16 indicates such a performance that the larger the value of the longitudinal-direction conversion transfer loss is increased, the higher the electrical balance is increased, and the ratio for converting the common-mode signal to the normal mode signal is small. In FIG. 16, symbol “A” shows such a characteristic that the additional winding 18 is not grounded, and symbol “B” indicates such a characteristic that one terminal of the additional winding 18 is grounded. As explained above, when the additional winding 18 is grounded, the electrical balance is lowered, and thus, the normal mode signal which is converted from the common-mode signal to be transferred is increased. As a consequence, there is such a problem that the sufficiently high common-mode rejection performance cannot be essentially obtained.